Inductively-coupled plasma ion source for use with a focused ion beam column with selectable ions

ABSTRACT

An inductively coupled plasma source having multiple gases in the plasma chamber provides multiple ion species to a focusing column. A mass filter allows for selection of a specific ion species and rapid changing from one species to another.

This application is a Continuation of U.S. patent application Ser. No.14/474,776 filed Sep. 2, 2014, which is a Continuation of U.S. patentapplication Ser. No. 13/312,704 filed Dec. 6, 2011, both of which arehereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to charged particle beam processingsystems, and in particular, to a charged particle beam system having aselectable ion species.

BACKGROUND OF THE INVENTION

Focused ion beam (FIB) systems are used in a variety of applications inintegrated circuit manufacturing and nanotechnology to create and altermicroscopic and nanoscopic structures. FIB systems can use a variety ofsources to produce ions. A liquid metal ion source (LMIS), for example,can provide high resolution processing, that is, a small spot size, buttypically produces a low beam current.

A typical system using a gallium LMIS can provide five to sevennanometers of lateral resolution. Such systems are widely used in thecharacterization and treatment of materials on microscopic to nanoscopicscales. A gallium LMIS comprises a pointed needle coated with a layer ofgallium. The needle is maintained at a high temperature while anelectric field is applied to the liquid gallium to extract ions from thesource.

A FIB system with a gallium LMIS can be used, for example, to image,mill, deposit, and analyze with great precision. Milling ormicromachining involves the removal of bulk material at or near thesurface. Milling can be performed without an etch-assisting gas, in amomentum transfer process called sputtering, or using an etch-assistinggas, in a process referred to as chemically-assisted ion beam etching.U.S. Pat. No. 5,188,705, which is assigned to the assignee of thepresent invention, describes a chemically-assisted ion beam etchingprocess. In chemically-assisted ion beam etching, an etch-enhancing gasreacts in the presence of the ion beam to combine with the surfacematerial to form volatile compounds. In FIB deposition, a precursor gas,such as an organometallic compound, decomposes in the presence of theion beam to deposit material onto the target surface.

In all of the processes described above, the function of the galliumions in the beam is to provide energy, either to physically displaceparticles on the work piece by sputtering or to activate a chemicalreaction of a molecule adhered to the surface. The gallium itself doesnot typically participate in the reaction. Gallium is used in the beambecause its properties, such as melting point, ionization energy, andmass, make it suitable to form into a narrow beam to interact withcommonly used work piece materials. There are disadvantages to using agallium LMIS. Gallium atoms implant into the work piece and, in manyapplications, produce undesirable side effects, such as changing theopacity or electrical properties of a work piece. Gallium can alsodisrupt the crystal structure in the area of bombardment. Also, toproduce a very narrow beam, the current in a beam from an LMIS must bekept relatively low, which means low etch rates and longer processingtimes.

While it would be desirable to use different ion species for differentapplications, liquid metal ion sources are limited in the type of ionsthey can produce. Only metal species that have suitable melting point,ionization energy, and mass can be used, and this limits the ion speciesavailable from an LMIS. While it would also be desirable to be able torapidly change ion species for different processing steps, changing theion species of an LMIS requires removing the source from the vacuumchamber and replacing it with a different source, which must thenundergo a time consuming preparation procedure. There are liquid metalalloy sources that provide ions of more than one type of metal. Thetypes of ions available from such sources are limited to combination ofmetals that form alloys having suitable properties. Some liquid metalalloy systems, such as the one described in U.S. Pat. No. 5,165,954 toParker, et al. for “Method for Repairing Semiconductor Masks &Reticles,” are used with a mass filter that separates the ions ofdifferent species so that the beam impacting the target comprises asingle species.

Some types of mass filters, such as an E×B filter or “Wien filter,” usean electric field and a magnetic field, perpendicular to the electricfield, that pass ions of the selected mass and energy through thefilter, while ions having other masses, or energies, are deflected intoa barrier. The deflection of an E×B filter depends on the energy of theions, and there is always some energy variation in the beam, so thefilter introduces chromatic aberration into the system, spreading thebeam and reducing its resolution. Non-uniform fields within the massfilter also contribute to beam aberration.

Plasma ion sources ionize gas in a plasma chamber and extract ions toform a beam that is focused on a work piece. Plasma ion sources, such asa duoplasmatron plasma ion source described by Coath and Long, “AHigh-Brightness Duoplasmatron Ion Source Microprobe Secondary Ion MassSpectroscopy,” Rev. Sci. Instruments 66(2), p. 1018 (1995), have beenused as ion sources for ion beam systems, particularly for applicationsin mass spectroscopy and ion implantation. Because of the energy spreadof ions extracted from the plasma chamber, the ions of a duoplasmatronsource cannot be focused to as small a spot as the ions from an LMIS.Duoplasmatron ion sources are used, for example, to implant ions over alarge area or for time-of-flight mass spectroscopy. In time-of-flightmass spectroscopy, ions in the primary beam sputter ions from thesurface, and the mass of each sputtered ions is determined by the timerequired for the sputtered ion to reach the detector. To obtain aprecise measurement, it is necessary to known precisely when the ion inthe primary beam impacts the surface. Many gases species consist ofmultiple isotopes having slightly different masses and because differentisotopes in the primary beam will reach the specimen at different times,mass filtering is used to separate isotopes, so that a single isotopereaches the specimen at a precisely known time.

Recently, inductively coupled plasma (ICP) ion sources have begun to beused in FIB systems. Innovations in ICP sources have reduced chromaticaberration, allowing for higher resolution processing, which opens newopportunities for ion beam processing, including imaging.

Many different types of gases can be used in a plasma ion source toprovide a variety of ions species, so the ion species can be optimizedfor different applications. For example, whereas helium ions are usefulfor imaging or light polishing, xenon ions provide higher milling ratesthat are useful for bulk processing. Plasma ion sources can produce ionsof many different species and at larger currents, but beam resolutionhas been limited. When a user wants to change ion species in a plasmasource, it is necessary to remove a first gas from the plasma chamberand replace it with a second gas. U.S. Pat. Pub. No. 2009/0309018 for“Multi-Source Plasma Focused Ion Beam System,” which is assigned to theassignee of the present invention and is hereby incorporated byreference, describes a system for providing multiple gases to the plasmachamber to provide different ion species for performing differentcharged particle beam operations. Unfortunately, it can take up to 30minutes to remove one gas from the plasma chamber and fill it with asecond gas. A gas inlet for a plasma ion source typically has a smallopening through which gas is supplied to maintain the pressure in theplasma chamber. Because the gas is used very slowly, the small openingto replenish the gas is very small. This makes for an unacceptably longtime to change out the gas for many applications that process a workpiece sequentially using different process gases.

FIG. 1 shows a typical prior art ICP ion source 100 for use with a FIBsystem such as the one described in U.S. Pat. Pub. No. 2009/0309018. Gasis provided to a plasma chamber 102 within a source tube 103 from anexternal gas feed line 104 through a gas filter 106 and then to acapillary tube 108 with a flow restriction 110. Energy is fed into theplasma chamber 102 from RF power supply 113 by antenna coils 114 andions are extracted through a source electrode aperture 116 in a sourceelectrode 118 by extractor electrode 120. A split Faraday shield 121reduces the capacitive coupling between the coil 114 and the plasma inchamber 102, in chamber 102 which reduces the energy spread of theextracted ions. Power supply 113 preferably drives the antenna 114 in a“balanced” manner, that is, the electrical phase shift across theantenna is adjusted to reduce modulation of the plasma potential asdescribed in U.S. Pat. Pub. No. 20080017319 of Keller et al. for a“Magnetically enhanced, inductively coupled plasma source for a focusedion beam system,” which is assigned to the assignee of the presentinvention and which is hereby incorporated by references. The balancedantenna preferably provides a null point in the radio frequency energyfield within the plasma, which reduces the energy spread of the ionsextracted from plasma chamber 102.

The gas conductance into and out of the plasma chamber 102 is throughthe flow restriction 110 in the capillary tube (at the top of the sourcetube 103) and the aperture 116 (typically less than ¼ mm in diameter) inthe source electrode 118. Pump 122 connected to gas supply line 104through valve 123 removes gas from plasma chamber 102 through capillary108 and gas supply line 104. An ion column pump (not shown) extracts gasfrom plasma chamber 102 through source electrode aperture 116. Multiplegas sources such as gas storage 130A, gas storage 130B, gas storage 130Cand gas storage 130D supply gas into gas supply line 104 throughcorresponding valves 131A through 131D. A beam voltage supply 132supplies a high voltage to the plasma in chamber 102 and an extractionvoltage supply 134 supplies a voltage to extraction electrode 120.Extracted ions or electrons are focused by focusing electrode 136.Additional details of the focusing column and sample chamber are notshown.

To remove a gas from the interior of the plasma chamber, the gas feedline 104 is pumped as shown to remove gas in the source tube above theflow restriction 110 in the capillary tube 108. The volume of the FIBsystem below the source electrode 118 may also be adequately pumpedusing the main chamber vacuum pump(s) (not shown).

Because both the source electrode aperture 116 and the flow restrictor110 have small diameters and correspondingly very low gas conductances,it is impossible to rapidly pump out the interior of the source tube103. This is a disadvantage, particularly for a production FIB systemwhere it is sometimes desirable to perform sequential process steps withdifferent ion species. First, it may take a much longer time to pump outa first process gas from the source tube 103 before the base pressure islow enough to introduce a second process gas. Insufficient purging ofthe gas can lead to contamination of the plasma through ionization. U.S.patent application Ser. No. 13/182,187 for “Methods and Structures forRapid Switching between Different Process Gases in anInductively-Coupled Plasma (ICP) Ion Source” describes plasma chamberdesigns that provide for rapidly changing gas in a plasma source byproviding an alternate path for gas to enter or leave the vacuumchamber.

Thus, providing high resolution beams of different ion species islimited by long gas exchange time or, in the case of a metallic alloysource, the metals present in the alloy which are typically limited bythe ability to create such an alloy based on material compatibility.

SUMMARY OF THE INVENTION

An object of the invention is to provide a FIB system that provides forrapidly switching between different ion species.

In accordance a preferred embodiment of the invention, an ICP ion sourceis provided for a FIB column. The plasma ion source contains more thanone type of gas to provide ions of different species. The ion speciesthat comprises the focused beam is selected from the multiple ionspecies leaving the source using a mass filter.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a cross-sectional schematic view of a prior art FIB systemusing an ICP ion source.

FIG. 2A shows a cross-sectional schematic view of a first embodiment ofa FIB system using an ICP ion source and having a mass filter to selectthe ion species that impacts the sample. FIG. 2B shows a cross-sectionalschematic view of a second embodiment of a FIB system using an ICP ionsource and having a mass filter to select the ion species that impactsthe sample.

FIG. 3 shows an embodiment of a mass filter that uses an aberrationcorrected Wien E×B mass filter.

FIG. 4 shows an embodiment of a mass filter that uses two electrostaticpole pieces, each with a separate electrical connection.

FIG. 5 shows an isometric quarter-cutaway view of an embodiment of anE×B mass filter used in some embodiments of the present invention.

FIG. 6 shows the side cross-section of cut line C-C of FIG. 5illustrating the magnetic circuit of an E×B mass filter used in someembodiments of the invention.

FIG. 7 shows a flow chart of the preferred method in accordance with thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred embodiment comprises an ICP ion source having multiple gasspecies within the source and a mass filter for selecting from themultiple ion species extracted from the source an ion species to impactthe target in a focused ion beam. The preferred embodiment provides avery broad range of mass and beam current choices for use across analmost limitless variety of applications. This differs from the massfiltering in, for example, time-of-flight mass spectroscopy, in which asingle species is selected from multiple isotopes of slightly differentmasses. The combination of ICP source and mass filter provides asolution to the problem of providing a high resolution beam togetherwith rapid switching between a wide choice of selectable ion species.The mass filter allows ions of different species to exit the sourceduring beam formation, with a particular species selected while otherspecies are rejected by means of the mass filter. Because the multiplegases are present in the plasma during operation, the ion species can bechanged rapidly in less than one minute, less than one second, less than0.5 second, or less than 0.1 sec, by simply changing the configurationof the mass filter.

A preferred mass filter is a Wien filter, also called an E×B filterbecause the beam travels through a region having a magnetic field and anelectric field perpendicular to the magnetic field. Other types of massfilters, such as a quadrupole, a sector instrument, or a sphericalcapacitor, can also be used. The ICP plasma source and E×B mass filter,when combined with a mixed source gas, provides a method for generatinga plasma comprised of ions from all source gases simultaneously. Thegases can be introduced into the plasma chamber in a fixed ratio from asource of premixed gas or in a selectable ratio from individual gassources. The mixed gas in the plasma chamber is not the result ofcontamination or of different isotopes of the same elements, butcomprises different elements, each in sufficient quantities in theplasma to form a beam for processing a work piece.

While the E×B filter is known to introduce chromatic and otheraberration into the beam, applicants have found that the energyvariation among ions exiting the ICP plasma source is sufficiently smallthat the aberration introduced in the mass filtered beam and thefocusing column is small, thereby providing a small spot size and highresolution processing. A preferred ICP produces ions of the same specieshaving energy spread of less than 40 eV, more preferably less than 20eV, and most preferably less than 6 eV. The combination of the ICPplasma source, the mass filter, and the ion column can provide an ionbeam having a spot size less than 1 micron, less than 500 nm, less than100 nm or less than 50 nm. The beam current at the work piece providedby the combination of the ICP and mass filter is typically between 1 pAand 10 μA. In one embodiment, the combination of ICP plasma source andmass filter is capable of providing a beam having a current of 10 μAwith a spot size of less than 200 μm, less than 500 nm, or less than 100nm. In another embodiment, the combination of ICP plasma source and iscapable of providing a beam having a current of 1 μA with a spot size ofless than 500 nm, less than 100 nm, or less than 50 nm. In anotherembodiment, the combination of ICP plasma source and is capable ofproviding a beam having a current of 10 nA with a spot size of less than200 nm, less than 100 nm, or less than 25 nm. In another embodiment, thecombination of ICP plasma source and is capable of providing a beamhaving a current of 1 pA with a spot size of less than 20 nm, less than10 nm, or less than 5 nm. The beam resolution is improved by variouscombinations of lens mode operation, the split faraday shield andbalanced antenna in the source and various techniques to reduceaberration in the mass filter. Depending on the resolution required fora specific application, not all of the resolution enhancing featureswill be used in every embodiment.

The E×B filter allows selection or rejection of one ion species from amixed source to carry out specific operations which benefit from use ofa specific ion, such as a light ion, a heavier ion, an inert ion, or areactive ion. One embodiment uses a mixed gas of very light, mid-weightand heavy ions, such as neon, argon and xenon, which allows for a widerange of ion masses to cover a broad range of applications.

Some embodiments provide for the composition and/or ratio of the gasesto be controlled by pre-mixing discrete gases externally in a commonplenum prior to introduction into the plasma source. The common plenumis then connected via the gas supply line to the ion source. In otherembodiments, the gases connect to the gas supply line from multipleindividual gas sources, each containing a single gas species.Embodiments allow a broad range of power and pressure settings, whichmay preferentially increase the performance of one desired species overanother while still maintaining ion species selection capability by useof the mass filter. For example different gases such as oxygen and xenonmay be mixed with variable stoichiometry and may be further optimizedthrough operation in different power and pressure regimes. In general,the lower limit of gas pressure is defined by the pressure correspondingto high voltage breakdown for the most limiting gas. It has been wellestablished through exhaustive characterization by Paschen that a uniquepower and pressure relationship exists for numerous, specific gasspecies. This relationship defines the acceptable range of operation ofthat gas at a prescribed voltage and within a certain pressure rangewhich will not result in high voltage breakdown of the gas Skilledpersons will recognize that various combinations of source gases,operating pressures and power settings are achievable for a wide rangeof gas species or combination of gas species within the same plasmasource and which are preferentially selectable either by mass filtering,or by careful regulation of power and pressure and in conjunction withfurther mass filtering.

In the above example, the most limiting gas is xenon which has a minimuminlet pressure requirement of approximately 1300 mbar before highvoltage breakdown occurs. Conversely, oxygen has a much lower limit ofapproximately 500 mbar. In addition, there is a minimum power settingwhich is necessary to create and sustain a plasma based on the sourcegas species. For example, xenon is capable of sustaining plasma at inputpowers as low as 15 W while oxygen requires a minimum input power ofapproximately 150 W. One can operate a mixture of both xenon and oxygeneither together with further mass filtering or independently simply bycareful regulation of power and pressure parameters. A typical range ofpower and pressure for operation of such a source gas combination mayrange from 25 W to 1000 W at a pressure of between 500 mbar and 3000mbar. A more preferred range of power and pressure may be from 100 W to600 W and 1000 mbar to 2000 mbar. Or conversely, a power setting of 25 Wto 100 W and pressure setting of 1500 mbar to 2000 mbar would result inthe ability to preferentially generate only xenon plasma, even in thepresence of oxygen, but without Paschen breakdown. In any event, thepresence of the mass filter allows for removal of any undesiredcomponent of the plasma beam after it is extracted and focused.Operation of a wide range of source gas species has been demonstrated,ranging from light ion species such as neon, to middle weight ionspecies such as argon and reaching to heavy ion species such as xenon.Other source gases such as nitrogen and oxygen have been demonstrated.It should therefore be possible to operate the plasma source and massfilter with any combination of these gases by the method described. Thisimplementation therefore allows great versatility, precise selection andease of operation for different applications ranging from rapid materialremoval to light ion imaging. Using the information above as guidance,skilled persons can select appropriate power and pressure relationshipsfor various gas combinations.

Regardless of how the multiple gases are supplied to the plasma chamber,embodiments provide for the immediate selection of ion species whereotherwise, an exchange of source gases would be required in order tochange the ion species required for a specific operation.

A benefit of some embodiments is that numerous gas mixtures, whetherpremixed or mixed in-situ in the gas supply line or plasma chamber,could be delivered. This is analogous to the ability to operate multiplealloy sources in one source assembly. For example, gas source couldprovide a mixture could be helium and xenon while another gas sourcecould provide a mixture of could be hydrogen and oxygen. In someembodiments, one or more mass filters could be set to pass more multipleion species to use, for example, for simultaneous imaging by light ionsand milling by heavy ions, as described in U.S. patent application Ser.No. 13/223,276 of Rue et al. for “Navigation and Sample Processing Usingan Ion Source Containing both Low-Mass and High-Mass Species,” which isassigned to the assignee of the present invention and which is herebyincorporated by reference.

An additional benefit of some embodiments is the ability of the ICPsource to deliver an extremely broad range of beam currents, therebytaking further advantage of the wide distribution of masses availableand their very different imaging and milling characteristics.

Applicants have demonstrated the capability of an embodiment using airas the gas in the plasma chamber. The plasma is predominantly composedof nitrogen ions while the remainder would be predominantly composed ofoxygen ions, with a small proportion of other elements such as argon andhelium.

FIG. 2A shows a FIB system 200 of the present invention. System 200 issimilar to system 100 of FIG. 1, with the addition of a mass filter 202below the ion source. As in FIG. 1, a preferred plasma source includes asplit Faraday shield 121 to reduce capacitive coupling between theantenna and the plasma and reduce the energy spread of the extractedions. A preferred plasma source also includes an antenna 114 driven in abalanced manner that adjusts the phase shift across the antenna tominimize or eliminate radio frequency modulation of the plasmapotential. Mass filter 202 is preferably an E×B filter, although othertypes of mass filter can be used. Mass filter 202 include electrode 204that provide an electric field and magnets (not shown) positioned aboveand below the plane of the paper to provide a crossing magnetic field.Connectors 206 provide electrical connections to electrodes 204 andprovide a mechanical connection to adjust the position of the electrode204. The fields are adjustable to select the mass of ions that passthrough the filter undeflected and pass through an aperture in the beampath; ions having masses other than the selected mass will be deflectedand will not pass through the aperture. While mass filter 202 is shownschematically, it could comprise a more complex mass filter or acompound filter having more than one region of E×B fields, separatedalong the beam axis. Valves 131A to 131D are adjusted to provide thedesired gas mixture into gas feed line 104 to provide gas to the plasmachamber 106. Unlike the prior art, in which a single gas is typicallysupplied to plasma chamber 102 at any time, in embodiments of theinvention, multiple ones of valves 131A to 131D may be open at the sametime to provide multiple gas species to the plasma chambersimultaneously. Valves 131A to 131D are preferably metering valves thatcontrol the ratio of gases into gas inlet 104.

FIG. 2B is similar to FIG. 2A except that some of the gas supplies 130Ato 130D feed through valves 131A to valve 131D to a mixing plenum 210,in which the gas is premixed before passing through valve 212 into gasfeed line 104 and into the plasma chamber 102. Gas supply 130E feedsdirectly into gas supply line 104. In various embodiments, any number ofsingle gas supplies and of mixing plenums can be combined.Alternatively, a mixed gas source 210 can be prepared in anotherlocation and connected to valve 212, eliminating source 130A to 130E atthe FIB system.

Mass filter 202 could comprise, for example, the mass filter shown inFIG. 3 and described in more detail in U.S. patent application Ser. No.13/089,991 for “Aberration Corrected Wien E×B Mass Filter with Removalof Neutrals from the Beam,” which is assigned to the assignee of thepresent invention and which is hereby incorporated by reference. FIG. 3illustrates an ion column 302 having an aberration-corrected mass filter304 having two stages, an upper E×B filter 306U and a lower E×B filter306L. Ions 312 are drawn from an ICP plasma chamber 313 by an extractionelectrode 315.

Ions 312 are then focused into a substantially parallel beam 310 byupper lens 348. In a completely parallel beam 310, the individual iontrajectories within the beam 310 may be extrapolated back to a virtualsource (not shown) at minus infinity along the optical axis 380. A“substantially parallel” beam is a beam for which the virtual source isnot necessarily at minus infinity, but for which the extrapolated iontrajectories still intersect the optical axis 380 at a location at leastthree times farther from the source tip (above or below) than theoverall length of the ion column 302. Upper E×B filter 306U includeselectrodes 314U, field termination plates 316U, and a magnetic fieldsource (not shown).

Electrodes 314U produce an electric field in the plane of the figure,indicated by arrow 320U (pointing from the positive electrode 314U atthe left towards the negative electrode 314U at the right—making theelectric force on a positive ion towards the right). The magnetic fieldsource produces a magnetic field coming out of the figure, indicated bycircle 322U (making the magnetic force on a positive ion towards theleft). Lower E×B filter 306L includes electrodes 314L, field terminationplates 316L, and a magnetic field source (not shown). Electrodes 314Lproduce an electric field, indicated by arrow 320L in the plane of thefigure, opposite in direction and equal in magnitude to electric field320U in upper E×B filter 306U. The magnetic field source in lower E×Bfilter 306L produces a magnetic field going into the figure, asindicated by cross 322L, opposite in direction and equal in magnitude tomagnetic field 322U in upper E×B filter 306U. Lower E×B filter 306L issymmetrical with upper E×B filter 306U, typically having an identicalstructure (rotated 180° and with a symmetry axis offset a distance 326)and producing opposite-direction and equal-magnitude electric andmagnetic fields.

Ions 312 include four different ion species as shown: low mass ions 330,lower middle mass ions 332, upper middle mass ions 334, and upper massions 336. The low mass ions 330, upper middle mass ions 334, and uppermass ions 336 strike a mass separation aperture plate 340 and do notpass through aperture 342 down to the lower lens 344. The lower middlemass ions 332 pass through both the upper E×B filter 306U and lower E×Bfilter 306L as shown. Ions 332 then pass through the mass separationaperture 342 and are focused onto substrate surface 112 by the lowerlens 344. In the prior art, E×B filters are typically tuned to pass thedesired ions (lower middle mass in this example) without deflection. Inthe embodiment of FIG. 3, the desired ions are deflected to pass throughaperture 342 and some of the undesirable ions (in this example, uppermiddle mass 334), along with the neutral particles, are undeflected andstrike aperture plate 340. Other undesirable ions are deflected too much(low mass 330) or too little (high mass 336) to pass through aperture342.

Neutral particles 346 are undeflected by the electric and magneticfields in the E×B mass filter 304 and thus pass straight through,striking the mass separation aperture plate 340 because the hole 342 inthe aperture plate 340 (which defines the exit axis of the E×B filter304) is offset a distance 326 from the entrance axis 380 of the E×Bfilter 304. Although the schematic of FIG. 3 does not make it clear thatthere is no path to substrate 112 for neutral particles that areundeflected by lens 348, the geometry of an actual system eliminatessuch a path by various means familiar to those in the art, such as anaperture at the entrance to upper E×B filter 306U and/or an aperturesomewhere in the column below mass filter 304. Commonly, the fieldtermination plates 316U and 316L may be configured with openings for theion beam to enter and exit through which are small enough to serve asapertures. Because the ions that impact aperture plate 340 are notfocused onto a point, the wear on the aperture plate caused by ionsputtering is spread over a wider area. Thus aperture plate 340 is lesslikely to have an unwanted hole sputtered through the plate by theblocked ions, so aperture plate 340 will last longer.

In a preferred embodiment, upper E×B filter 306U and a lower E×B filter306L are of the type describe in U.S. patent application Ser. No.13/089,875 For “Wide Aperture Wien E×B Mass Filter,” which is assignedto the assignee of the present invention and which is herebyincorporated by reference. Such an E×B filter includes magnetic polesthat extend beyond the aperture to reduce fringe effects and provides anadjustable electric field having components both parallel andperpendicular to the magnetic field. The adjustable electric field cancompensate for the non-ideal configuration of the physical electrodes toprovide a wide optical aperture. The adjustable electric field can alsoprovide in some embodiments the capability for both X-Y beam deflection,which can be used for beam alignment. The adjustable electric field canalso provide in some embodiments beam stigmation, which can be used forcorrecting some of the aberrations induced by the mass filter.

FIG. 4 shows an embodiment of a mass filter 400 that uses twoelectrostatic pole pieces 430R and 430L, each with a separate electricalconnection. Each of E×B filters 306U and 306L can comprise an E×B filterof the design shown in FIG. 4. Beam motion is assumed to be generallyalong the Z-axis (perpendicular to the plane of the figure). A firstelectric pole, 430R, is positioned on the +X-axis at a distance LX1 fromthe Y-axis (vertical centerline) with an applied voltage VA. A secondelectric pole 430L is positioned on the −X-axis at a distance −LX1 fromthe Y-axis with an applied voltage VC. The electric pole faces areoriented parallel to the Y-Z plane. The values of VA and VC would bechosen based on standard Wien filter operating considerations—see below.

There are two magnetic poles, 404U and 404L, positioned with their polefaces oriented parallel to the X-Z plane and at positions ±LY on the +Yand −Y-axes. The magnetic pole faces extend beyond the region defined bythe electric plates and magnetic pole pieces. Either coils and/orpermanent magnets may energize the pole pieces 404U and 404L, generatinga magnetic field parallel to the Y-axis. The pole pieces 404U and 404Lare fabricated from ferrite or some similar resistive magnetic material,typically in a resistivity range from 10⁶ to 10⁸ ohm-cm. The upper(+Y-axis) magnetic pole has two electrical connections, one at the +Xend (VB1) and the other at the −X end (VB4). The lower (−Y-axis)magnetic pole also has two electrical connections, one at the +X end(VD1) and the other at the −X end (VD4). As described in U.S. patentapplication Ser. No. 13/089,875, voltages can be applied to the magneticpole pieces to correct astigmatism induced by the crossed electric andmagnetic fields in the mass filter. The voltage on the magnetic polescan also be applied to provide for electrostatic deflection of the beamin a direction parallel to the electric field or in a direction parallelto the magnetic field, which deflection can be used to align the beam inthe column.

Some embodiments of the invention include a mass filter shown in FIGS. 5and 6 and as described in U.S. patent application Ser. No. 13/111,634for “Method and Structure for Controlling Magnetic Field Distributionsin an E×B Wien Filter” which is assigned to the assignee of the presentinvention and which is hereby incorporated by reference. The mass filterof FIGS. 5 and 6 provides a structure and method for mechanicallyadjusting the magnetic field distribution and the entrance and exitapertures to achieve better matching between the electric and magneticfields thereby equalizing the opposing electric and magnetic forcesthroughout the length of the E×B filter, including near the end caps andwithin the entrance and exit apertures.

As described in U.S. patent application Ser. No. 13/111,634 the ratio ofthe gap reluctance to the yoke reluctance should equal the ratio of theleak reluctance (i.e., the reluctance between the edges of the polepiece and the end caps) to the reluctance of the spacers. This designprovides closer matching of the ratios by means of magnetic shims in theend caps and magnetic plug shims in the end rings, thereby providingflexibility in meeting the required B/E ratio throughout the length ofan E×B than is possible merely by means of materials choices for thespacers or for the end caps.

FIG. 5 is an isometric quarter-cutaway view of an E×B mass filter 500that can be used in an embodiment of the present invention. Section C-Cis illustrated in FIG. 6. A magnetic pole piece 502 is attached to aceramic insulator 504, which is attached to magnet 506, typically aneodymium-iron-boron (NdFeB) or samarium-cobalt (SmCo) alloy magnet, orother similar high strength permanent magnet. In alternative embodimentswithin the scope of the invention, electromagnet coils may besubstituted for the permanent magnets 506 shown here. Magnets 506(typically in a pair—only one is visible in the cutaway view 500) areattached to yoke 508, typically comprised of a relatively highsaturation magnetic material, such as nickel-iron (e.g., NiFe43 orNiFe48).

In FIG. 5, the ion beam to be mass-separated would enter mass filter 500through aperture 524 in entrance ring 530 mounted to entrance end cap522. The various mass-separated ion beams would exit mass filter 500through exit aperture 528 in exit ring 532 mounted to exit end cap 526.In general, the angular deflection of the non-selected ion species(i.e., those species not wanted in the ion beam to be focused on asample) will be deflected along the E-field axis—from the lower left tothe upper right of FIG. 5. This deflection in the majority of cases willbe small enough that these non-selected ion beams will pass through exitaperture 528, to be blocked by a mass separation aperture (not shown)below the E×B mass filter. The selected ion beam will pass approximatelythrough the center of exit aperture 528 and then through the massseparation aperture to be focused on the specimen. The magnetic fieldwhich performs the mass-separation of the ions beams is generatedbetween the inner faces of the two pole pieces 502. These “gap fields”624 are shown in FIG. 6.

Perpendicular to the B-field 624 (FIG. 6), an electric field isestablished between a pair of electrodes 542 which in the preferredembodiment shown are pressed inwards by mounting screws 544 (threadedinto yoke 508) that push against insulators 546. Electrical connectionsto electrodes 542 are effected through rods 548 extending radiallyoutwards through clearance holes in the yoke 508 and housing 518, andhaving corona-prevention balls 554 at the outer ends. The electrode andpole piece configuration shown corresponds to that shown in U.S. No.4,789,787 issued Dec. 6, 1988 (see FIGS. 4A and 4B therein), which isincorporated by reference. The entrance end cap 522 has a thickenedouter ring 592. Outer ring 592 has a radial slot 550 which serves toincrease the axial reluctance of the outer ring. Magnetic shims 590 maybe inserted into slot 550 to reduce the axial reluctance of the outerring 592. Since the number, positions, and permeabilities of shims 590are mechanically adjustable, it is possible to vary the reluctance ofthe outer ring 592 over a wide range in small increments, therebyenabling a much finer adjustment of the end cap reluctance than ispossible in prior art E×B mass filters. Similar considerations apply tothe radial slot 556 in exit end cap 526 having thickened outer ring 594and magnetic shims 596.

A housing 518 encloses the yoke 508, with a clamping ring 520 (held downby screws 557) that compresses together end caps 522 and 526, and yoke518. Below the E×B mass filter is an X-Y beam deflector 582 used forcorrection of beam deflection errors arising from the E×B mass filter500. Mass filter 500 provides much closer matching of the B-field andE-field distributions than a conventional mass filter. This bettermatching is enabled by mechanically variable reluctances.

FIG. 6 is a side cross-section C-C of mass filter 500 illustrating themagnetic circuit of an E×B mass filter showing a preferred embodiment ofthe present invention. Arrows 602-634 illustrate the magnetic fluxdistribution within gaps, magnetic materials, and magnets. B-field 624is “seen” by the ion beams passing through the E×B mass filter andgenerates the magnetic force on the ions which is generally opposite indirection and similar or identical in magnitude to the force induced bythe E-field between the two electrodes 542 (one shown in FIG. 5). Fluxes622 and 626 are between the pole pieces 502 and the yoke 508, passingthrough the magnets 506 and insulators 504. Within the entrance end cap522 and entrance ring 530, fluxes 602 and 606 always flow to the left,corresponding to the direction of both the return flux flowing aroundthe outside of the magnetic circuit, as well as connecting leakagefluxes 628 and 630 to the outer ends of magnets 506. Similarly, withinexit end cap 526 and exit ring 532, fluxes 608 and 612 also always flowto the left, corresponding to the direction of both the return fluxflowing around the outside of the magnetic circuit, as well asconnecting leakage fluxes 632 and 634 to the outer ends of magnets 506.Fluxes 614 and 618 connect between the entrance end cap 522 and the yoke508, passing through the outer ring 592 of end cap 522 and also anymagnetic shims 590 within radial slot 550. Fluxes 616 and 620 connectbetween the exit end cap 526 and the yoke 508, passing through the outerring 594 of end cap 526 and also any magnetic shims 596 within radialslot 556.

Now consider the B-fields inside the entrance aperture 524 withinentrance ring 530. Unlike the prior art mass filters described above, ina mass filter according to the present invention, the magnetic fieldorientation and magnitude within the entrance and exit apertures can beadjusted to match the electric and magnetic fields thereby equalizingthe opposing electric and magnetic forces throughout the length of theE×B filter. As shown in FIG. 6, the numbers, positions andpermeabilities of magnetic shims 590 have been adjusted to cancel outthe B-field within entrance aperture 524, thus no flux is shown acrossaperture 524.

As discussed above, the function of the entrance end cap 522 andentrance ring 530 is to terminate both the B-field and the E-field,ideally with approximately the same rate of decrease as a function ofdistance along the axis of the E×B mass filter, thereby preserving thecorrect B/E ratio. Since the entrance end cap 522 and entrance ring 530have good electrical conductivity, the E-field tends to be terminatedfairly abruptly and generally with essentially no E-field withinaperture 524. For correct E×B operation, the B-field strength should beproportional to the E-field (with the same ratio everywhere on-axis), sothe B-field should drop off to approximately zero strength withinaperture 524, as well. Similar considerations apply to the B-fieldinside aperture 528 in exit ring 532 mounted to end cap 526. Unlike theprior art mass filters described above, in preferred embodiments of thepresent invention, the B-field within entrance and exit apertures 524,528 can be made to drop to approximately zero strength in proportion tothe E-field. Using the invention described herein, along with suitablechoices of material with which to fabricate entrance end cap 522, exitend cap 526, entrance ring 530, exit ring 532, magnetic shims 590 andmagnetic shims 596, it is now possible to cancel out the B-field withinthe entrance 524 and exit 528 apertures to less than one percent of themaximum B-field in the gap between pole pieces 502. Exemplary suitablematerials for the end caps, rings, and shims include materials withmoderate permeabilities such as 400-series stainless steel, inparticular alloy SS430.

Mass filter 500 enables a mechanical method and structure for achievinggood matching of the E-fields and B-fields at the E×B entrance and exit.Better matching reduces aberrations in the E×B mass filter and alsoenables better beam alignment exiting from the mass filter as isfamiliar to those skilled in the art. A preferred embodiment of FIBsystem may include a mass filter having any combination of the featuresof the mass filters shown in FIG. 3 to FIG. 6.

FIG. 7 shows a preferred method in accordance with the presentinvention. In step 702, the plasma chamber is filled with a mixture ofgases. For example, the gases might include helium and xenon gases,where helium could be used for imaging or light polishing, while xenoncould provide relatively high milling rates for bulk processing. Thegases could be pre-mixed in a container; the gases could come fromdifferent containers, or any combination of the two. In step 704, aplasma is ignited in the plasma chamber. In step 706, a first voltage isapplied to the electrode of the mass filter to produce an electric fieldin the mass filter and a first current is applied to the electromagnetto produce a magnetic field in the mass field, the magnetic field beingperpendicular to the electric field. The electric field and the magneticfield are tuned to pass a first ion species and to deflect the other ionspecies into a blocking structure.

In step 708, a voltage is applied to the extractor electrode to extractions form the plasma chamber. In step 710, a work piece is processed byfocusing the extracted first ion species. While ions of multiple speciesare extracted from the plasma source, the mass filter will pass only thefirst ion species to process the work piece. The ions selected toprocess the work piece can be non-reactive ions that remove material bysputtering, that is, momentum transfer to the substrate, or the ions canactivate a precursor gas to etch or deposit material. Alternatively, theions in the beam can themselves be reactive and include can etch withoutsputtering. For example, ions in the beam may have insufficient energyto sputter material and but may have sufficient energy to initiate achemical reaction with the substrate material to form a volatilecompound. Such low energy reactive ions reduce damage to the work pieceand can selectively remove specific materials from the work piece. Inother embodiments, reactive ions may have sufficient energy to sputteras well as to induce a chemical reaction. Operations that the ionspecies can perform include imaging, fine milling, coarse milling, ordeposition.

When one step of the sample processing is complete, the extractionvoltage is removed from the extractor electrode in step 712. In decisionblock 714, it is determined whether processing is complete. Ifadditional processing is required, the process returns to block 706. Ifa different species of ion is required for the next operation, a secondvoltage is applied to the electrode of the mass filter to produce asecond electric field in the mass filter and a second current is appliedto the electromagnet to produce a second magnetic field in the massfield, to select a second ion species. The work piece is processed instep 710 using the second ion species. Multiple ion species areextracted from the ICP ion source during each processing step, but onlythe selected species passes through the mass filter and impinges on thework piece.

In decision block 714, if is determined that an additional processingstep is required, the process repeats in step 706. If a differentspecies of ion is required for the next operation, a third voltage isapplied to the electrode of the mass filter to produce a third electricfield in the mass filter and a third current is applied to theelectromagnet to produce a third magnetic field in the mass field, toselect a third ion species. The work piece is processed in step 710using the third ion species. As in each processing step, multiple ionspecies are extracted from the ICP ion source during each processingstep, but only the selected species passes through the mass filter andimpinges on the work piece. If necessary, an additional gas species canbe added to the plasma chamber from a source of single gas or mixed gas.

As describe above, in some embodiments, an ion species may be selectedwithout using a mass filter by adjusting the gas pressure in the plasmachamber and the power so that one or more species in the gas mixtureforms a plasma and one or more species in the gas mixture does not forma plasma. In such embodiments, a mass filter can still be used toprevent unwanted components that exit the plasma chamber from reachingthe specimen. The pressure of the gasses in the chamber and/or the powercan be altered to change which gases for the plasma, thereby changingthe type of ions that can be extracted. In some embodiments the ionsproduced are controlled by the pressure of the gases in the plasmachamber and the radio frequency power inductively applied into theplasma chamber to control which gases form a plasma. For example, amethod of processing a work piece with focused ion beam system having aninductively coupled plasma ion source, can comprise providing a plasmachamber including multiple gases; inductively coupling radio-frequencypower from an antenna into the plasma chamber, the radio-frequency powerbeing sufficient to maintain a plasma of a first group of at least oneof the multiple gases and insufficient to maintain a plasma from asecond group of at least one of the multiple gases; extracting ions ofthe first group from the plasma chamber; and focusing the extracted ionsonto a work piece. Some embodiments further comprise configuring a massfilter to pass one or more gases from the first group and to stop ionsother than the one or more gases from the first group.

Some embodiments further comprise altering the radio frequency powerinductive coupled in to the plasma chamber or altering the pressure ofone or more of the gases in the plasma chamber so that at least one ofthe gases in the first group does not form a plasma or so that one ormore gases in the second group does form a plasma. Some embodimentsfurther comprise extracting from the plasma chamber a group of ionsdifferent from ions of the first group.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

We claim as follows:
 1. A charged particle beam system, comprising: aninductively coupled plasma ion source; one or more gas sources forproviding multiple gases to the plasma ion source to produce multipleion species simultaneously from the plasma ion source; a mass filter toselect an ion species from the multiple ion species produced by theplasma ion source; and focusing optics to produce a focused beam of theselected ion species at a target, the beam having a submicron diameterat the target.
 2. The charged particle beam system of claim 1, whereinthe mass filter comprises an E×B filter.
 3. The charged particle beamsystem of claim 1, wherein the inductively coupled plasma ion sourceincludes a split Faraday shield.
 4. The charged particle beam system ofclaim 1, wherein the inductively coupled plasma ion source includesbalanced antenna.
 5. The charged particle beam system of claim 1,wherein the one or more gas species comprises two gas species.
 6. Thecharged particle beam system of claim 1, wherein the mass filtercomprises an aberration corrected E×B mass filter.
 7. The chargedparticle beam system of claim 6, wherein the aberration corrected E×Bmass filter has multiple stages, a first E×B filter stage and at least asecond E×B filter stage.
 8. The charged particle beam system of claim 1,wherein said mass filter includes at least two electrostatic polepieces, each having a separate electrical connection.
 9. The chargedparticle beam system of claim 1, wherein said mass filter furthercomprises mechanically adjustable magnetic field distribution andentrance and exit apertures.
 10. The charged particle beam system ofclaim 9, wherein said mass filter further comprises least two magneticpoles, each magnetic pole having two electrical connections, oneelectrical connection at each end of each magnetic pole.
 11. The chargedparticle beam system of claim 1, in which the focusing optics focus amass filtered ion beam to a spot size of less than 100 nm at a beamcurrent of 10 μA.
 12. The charged particle beam system of claim 1, inwhich the focusing optics focus a mass filtered ion beam to a spot sizeof less than 50 nm at a beam current of 1 μA.
 13. The charged particlebeam system of claim 1, in which the focusing optics focus a massfiltered ion beam to a spot size of less than 25 nm at a beam current of10 nA.
 14. A method of processing a work piece with focused ion beamsystem having an inductively coupled plasma ion source, comprising:extracting multiple ion species simultaneously from a plasma chamber ofan inductively coupled plasma ion source; configuring a mass filter toselect one of the multiple ion species extracted from the plasmachamber; focusing the selected one of the ion species onto a work pieceto execute a first process on the work piece; configuring the massfilter to select another of the multiple ion species extracted from theplasma chamber; and focusing the other one of the ion species onto awork piece to execute a second process on the work piece.
 15. The methodof claim 14 in which the mass filter comprises an E×B filter and inwhich configuring the mass filter comprises applying a specified voltageto the electrode in the mass filter and a specified current to theelectromagnet in the mass filter.
 16. The method of claim 15 in whichextracting multiple ion species simultaneously from a plasma chamber ofan inductively coupled plasma ion source includes extracting multipleion species simultaneously from a plasma chamber of an inductivelycoupled plasma ion source having a split faraday shield.
 17. The methodof claim 15 in which extracting multiple ion species simultaneously froma plasma chamber of an inductively coupled plasma ion source includesextracting multiple ion species simultaneously from a plasma chamber ofan inductively coupled plasma ion source having a balanced antenna. 18.The method of claim 15 in which: extracting multiple ion speciessimultaneously includes extracting helium and argon; configuring a massfilter to select one of the multiple ion species extracted from theplasma chamber including configuring the mass filter to select argonions; and focusing the selected one of the ion species onto a work pieceto execute a first process on the work piece includes focusing a beam ofargon ions having a current of greater than 1 μA onto a spot having adiameter smaller than 50 nm.
 19. The method of claim 14 in whichconfiguring a mass filter to select one of the multiple ion speciesextracted from the plasma chamber or configuring the mass filter toselect another of the multiple ion species extracted from the plasmachamber includes configuring the mass filter to select more than one ionspecies.
 20. A method of processing a work piece with focused ion beamsystem having an inductively coupled plasma ion source, comprising:providing a plasma chamber including multiple gases; inductivelycoupling radio-frequency power from an antenna into the plasma chamber,the radio-frequency power being sufficient to maintain a plasma of afirst group of at least one of the multiple gases and insufficient tomaintain a plasma from a second group of at least one of the multiplegases; extracting ions of the first group from the plasma chamber; andfocusing the extracted ions onto a work piece. 21.-23. (canceled)